Current practice is to reduce the as-mined cobalt ore particle size by a combination of crushing and grinding. The ground ore is then added to large leaching tanks containing sulfuric acid. A reductant, most commonly sulfur dioxide, is then added to the tank to reduce insoluble trivalent cobalt to soluble divalent cobalt.
At the Shituru plant, 0.8 tonnes of sodium metabisulfite (SBMS, Na2S2O5) and 1.2 tonnes of copper powder were required per tonne of cobalt produced. These consumption figures make cobalt extraction expensive as these reagents comprised 47% of the operating costs for cobalt production. (M. D. Mwema, M. Mpoyo, and K. Kafumbila, Use of sulfur dioxide as reducing agent in cobalt leaching at Shituru hydrometallurgical plant, Journal of The South African Institute of Mining and Metallurgy volume 102, issue 1, 2002, p. 1-4)
To reduce these costs, gaseous sulfur dioxide has been trialled as a replacement for sodium metabisulfite (‘SMBS’) and copper powder. Tests were performed at 40° C. using ore ground to 80%<74 μm. It was found that by sparging SO2 into the slurry cobalt recovery reached 86% after three hours. However, the sulfur dioxide also reduced iron and manganese within the ore, rendering them soluble in the acidic solution. This increased solubility necessitates further processing to remove these elements from solution giving an increasingly complex, and therefore expensive, flowsheet.
Miller (G. Miller, Design of copper-cobalt hydrometallurgical circuits, Metallurgical Plant Design and Operating Strategies (MetPlant 2008), AusIMM, p. 447-460) notes that “[i]n an acid solution the SMBS disassociates to form SO2 (aq) which lowers the Eh in solution and reduces the cobalt oxidation state. SMBS is costly and is only partially utilised with side reactions producing sulfuric acid—particularly in the presence of manganese ions in solution.” Additionally, “newer projects are considering the use of liquefied SO2” to remove some of the problems surrounding the direct use of SO2 in smelter off gas.
The paper by Miller also examines the removal of impurities from the acidic leach solution, notably iron, manganese, calcium and zinc all of which require removal prior to cobalt concentration and recovery. In this paper, it is stated that                “Iron removal has been undertaken for many years in many hydrometallurgical process plants. The classic method is air oxidation to iron (III) and precipitation with lime and or limestone. All the current and previous Zambian and DRC project use this basic method. However the older style plants all suffer from the usual problems of:                    downstream gypsum precipitation and            fouling of process equipment and pipes.”                        
Miller also noted that for manganese removal                “The process used to date in Zambia and DRC has been a combined iron and manganese precipitation. This has removed the copper and some zinc; but also co-precipitated significant cobalt which has been lost.”        
The above summary shows that the acid system for leaching of cobalt has considerable problems, most notably the requirement to remove a suite of impurities by a variety of different methods without significant cobalt losses before cobalt can be recovered.
The method of the present invention has as one object thereof to overcome the abovementioned problems associated with the prior art, or to at least provide a useful alternative thereto.
Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
The discussion of the background art is included exclusively for the purpose of providing a context for the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was common general knowledge in the field relevant to the present invention in Australia or elsewhere before the priority date.